1. Field of the Invention
The present invention relates generally to an arrangement and method for molding a front or back mold half which is used, with a second complementary back or front mold half, for subsequent molding of a soft contact lens such as a hydrogel soft contact lens.
More particularly, the subject invention pertains to an arrangement and method as described for molding a front or back polystyrene mold half in a manner which is designed with heat flow considerations in mind to minimize the molding cycle time, which has been reduced to approximately three to six seconds, depending upon the wall thickness, while producing uniformly acceptable, optical quality mold halves.
2. Discussion of the Prior Art
U.S. Pat. No. 4,565,348 to Larsen discloses a typical prior art approach to manufacturing mold halves similar to the present invention. Pursuant to this prior art approach, the mold halves are molded as a set of eight mold halves carried on a frame in a 2.times.4 array. FIG. 3 of the Larsen patent illustrates a molded frame carrying a 2.times.4 array of concave front or female mold halves, while FIG. 5 therein shows a molded frame carrying a 2.times.4 array of convex back or male mold halves. The cluster assembly of the frame and molds is manufactured by injection molding the assembly as one piece with the molds being secured within an outer rectangular frame by small struts extending between the frame and the molds. The height of the frame is such that the surfaces of the molds are protected from scratching and mechanical damage during handling, and the frame in general has a shape facilitating stacking and handling. This prior art approach of molding such polystyrene mold halves in a cluster assembly typically takes approximately twenty-four (24) seconds, which is considered to be too long for the efficient production of such polystyrene mold halves. In contrast thereto, pursuant to the present invention, the molding of such polystyrene mold halves takes approximately three to six seconds, depending upon the wall thickness.
A complementary pair of front and back mold halves is used in the production of hydrogel contact lenses by direct molding of a monomer mixture wherein the mixture is dissolved in a non-aqueous, water-displaceable solvent. The mixture is initially placed in a two part mold consisting of the front and back mold halves which define the shape of the final desired hydrogel (i.e. water-swollen) lens. After a dosing step in which the front concave mold halves are substantially filled with the polymerization mixture, the concave front mold halves are covered with the back mold halves in a manner in which no air bubbles are trapped beneath the back mold halves, which are brought to rest on the concave front mold halves properly aligned and without distortion. This is preferably performed with back mold halves which are put on as individual units on the pools of polymerizable mixture held in the front concave mold halves. Accordingly, prior to the mold covering step, the plurality of back mold halves are separated from the frame by breaking or cutting. The back mold members are preferably held by a mechanical device while they are separated from the frame and which thereafter is used to guide them down and place them all simultaneously on each of the concave front mold halves containing the polymerizable mixture. The monomer/solvent mixture is then subjected to conditions whereby the monomer(s) polymerize, such as irradiation with actinic visible or ultraviolet radiation, to thereby produce a polymer/ solvent mixture in the shape of the reduced final size of the desired hydrogel lens.
After the polymerization process is completed, the two halves of the mold are separated (called demolding), typically leaving the contact lens in the front mold half, from which it is subsequently displaced. The front and back mold halves are used for only a single molding, after which they are disposed of. After the polymerization is complete, the solvent is displaced with water to produce a hydrated lens the final size and shape of which are quite similar to the size and shape of the original molded polymer/solvent article. The direct molding of hydrogel contact lenses is disclosed in U.S. Pat. No. 4,495,313 to Larsen, U.S. Pat. No. 4,680,336 to Larsen et al., U.S. Pat. No. 4,565,348 to Larsen, and U.S. Pat. No. 4,640,489 to Larsen et al., the entire disclosures of which are hereby incorporated by reference in this patent application.